The invention relates generally to devices for vectorially combining electrical signals from two or more sources, and more specifically concerns devices known as "resolvers" which vectorially sum the output signals from two or more magnetometers mounted in a vehicle, such as an aircraft, in such a way that the resultant signal is a function of the error in heading of the vehicle.
The invention concerns the design of resolvers which are generally lighter and more economical to fabricate than the conventional electromagnetic type. In aircraft or other vehicles, it is desirable to install the compass or other magnetic heading reference at a point remote from the pilot's instrument panel to avoid stray magnetic fields from the engine and from other instruments. In the situation described here, two or more inductive magnetometers are mounted at a remote location in such a way that their sensitive axes lie in a normally horizontal plane and are disposed at an angle with respect to each other. In the simplest case, which will be used as an example, two magnetometers are mounted so that their sensitive axes lie in a horizontal plane, when the aircraft is in normal flight, and at 90.degree. with respect to one another--one axis parallel to the direction of flight and the other at a right angle to it.
The magnetometers used are of the "flux gate" inductive type, which produce an electrical output signal proportional to the intensity of that component of the earth's magnetic field which is parallel to the sensitive axis of the magnetometer. The basic output signal is an amplitude-modulated AC carrier signal whose frequency is twice that of the excitation voltage used to excite the magnetometer, and whose phase relation to the excitation voltage indicates the polarity of the signal.
When this signal is processed by a suitable phase-sensitive demodulator, it is converted into a DC signal whose amplitude is proportional to the intensity of that component of the magnetic field parallel to the magnetometer's sensitive axis and whose polarity indicates the direction of the field with respect to the magnetometer.
Thus, when one of these magnetometers is rotated about a vertical axis, while its sensitive axis is maintained in a horizontal plane, the electrical output from the demodulator will produce one cycle of a sin wave for each 360.degree. of rotation. If two such magnetometers A and B (FIG. 1) are mounted in the aircraft as previously described, and the aircraft is rotated about its vertical axis, the electrical signals from the demodulators of the respective magnetometers will produce sinusoidal waveforms A and B which will pass through one cycle for each 360.degree. of rotation of the aircraft, and these sin waves will have a 90.degree. phase relation to each other as shown in FIG. 1.
The problem addressed by this invention is to vectorially sum these signals from the individual magnetometers in such a way that the resultant signal will be a sinusoidal function of the heading of the vehicle, and that its phase may be manually adjusted to cross the zero axis at any desired heading of the vehicle, so that it may be used as a heading error signal for use with an automatic pilot or with a manual steering error indicator.
FIG. 2 illustrates typical prior art in which the AC output voltages of the two magnetometers 11 and 12 are applied to the two mutually perpendicular stationary field windings 13 and 14, respectively, of a electromagnetic resolver 15. It can be see that, as the vehicle and the magnetometer assembly is rotated in the earth's magnetic field, the AC signal voltages from the two magnetometers 11 and 12 are proportional to the sin and cosine of the magnitude of the horizontal component of the earth's magnetic field. When these signal voltages are applied to the stationary windings 13 and 14 of the resolver, a composite AC magnetic field is generated within the resolver. The direction of this composite field will rotate through the same angle through which the magnetometer rotates with respect to the earth's magnetic field, and the rotatable winding 16 can be positioned by means of a calibrated dial 17 to produce a null output voltage for any particular heading of the vehicle. This voltage V.sub.o can then be used as a heading error signal in an autopilot or a visual heading error indicator.
FIG. 5 of U.S. Pat. No. 4,006,631 shows another example of prior art in which a resistive potentiometer is used for the resolver. Either the AC signal voltages or the demodulated DC signal voltages from the magnetometers are applied to diametrically opposite points on the continuous winding of the potentiometer. Two diametrically opposite movable contacts can be positioned by a calibrated dial to select the vehicle heading at which a null output voltage is obtained. In this device the potentiometer must have a continuous 360.degree. winding, and the winding must have a nonlinear relation between resistance and angle in order to use an evenly divided dial for adjusting the desired heading.
It is an object of this invention to provide devices known as resolvers for vectorially summing the output signals from two or more magnetometers mounted on a vehicle, such as an aircraft, to produce a resultant signal which is a function of the error in heading of the vehicle.
It is another object of this invention to provide resolvers which are generally lighter and more economical to fabricate than the conventional electromagnetic type.
A further object of this invention is to vectorially sum the signals from individual magnetometers mounted on a vehicle in such a way that the resultant signal will be a sinusoidal function of the heading of the vehicle.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.